Glossary
Reactive oxygen species.
Direct answer
Reactive oxygen species are highly reactive compounds of oxygen, such as superoxide anion, hydrogen peroxide and the hydroxyl radical. They form continuously in mitochondria during energy production and are made deliberately by defence cells. In small amounts they serve as signals; in excess they alter fats, proteins and genetic material.
Reactive oxygen species are widely assumed to be harmful by nature. The body needs them: phagocytes deploy them against pathogens, and cells use them to steer adaptive processes. The problem is not their presence but a lasting excess over protective systems. That imbalance is precisely what oxidative stress names.
In horses, muscle mass and oxygen uptake during exercise are both remarkable. Production of reactive species during hard work is correspondingly large, and so are the protective systems. Comparison with humans often fails because an equine blood value means nothing without the workload and the sampling time attached to it.
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